The principal aspects of syntheses and some reactions of organic compounds at high pressures are reviewed. 1. Introduction 2. Apparatus 3. Ionic Reactions 3.1. Condensation Reactions 3.2. Nucleophilic Reactions 3.3. Electrophilic Reactions 3.4. Additions and Eliminations 3.5. Rearrangements 3.6. Miscellaneous Reactions